Caledonia yawl project: shaping the centreboard foil profile

This part of the project was quite simply described by Ian Oughtred in his book “glued lapstrake boat building” as just rounding over the leading edge and shaping a taper for the trailing edge.

Knowing the density of water and the impact of good foil profiles for sailing performance I needed just a little more of a systematic approach. Something between CNC machining a NACA 0080 profile and sculpting it by eye.

I found two approaches that appealed to me. One was to map out the NACA profile in Excel and print out a template with a 40mm offset for a router bit and cut out the profile with the router using a plywood Guide.

Full instructions and a link to download the excel file here.

The problem was that in printing you need to calibrate for both the x & y axis. In my case when I got back to the boat shed I found that the y axis was off by 10% in the printouts. So I could not use it without reprinting. Additionally my centreboard tapers from the top to the bottom so I would need to cut several templates for the changing chord length.

Instead of going back and calibrating the print area, I resorted to a good system described in a paper by German epoxy company Gurit on building strongest & stiffest centreboards.

Gurit Guide to Wooden Foil Construction Figure 5
Gurit Guide to Wooden Foil Construction Figure 5
Testing the kerf method on an offcut from the centreboard.
Cutting the trailing edge kerfs on the centreboard
cutting the first leading edge kerfs on the centreboard

The method describes dividing the  chord of the centreboard into seven equal sections. Since the Caledonia Yawl centreboard tapers from 400mm to 230mm I did this at both ends for each chord length and then drew the lines to join them.  This allowed me to cut the kerfs to match the changing slope of the leading edge and the trailing edge to keep the same profile along the length of the board.

The other challenge is to kerf the taper of the maximum width of the centreboard as the chord gets shorter at the bottom.

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